1. Field of the Invention
This invention relates to magnetic bubble domain devices, and in particular, methods and processes for fabricating such devices. The invention is directed primarily to a process having only one high resolution pattern step which permits improved yield to devices.
2. Prior Art
Circuit elements of magnetic bubble domain devices consist of patterns of high permeability, soft magnetic material (usually permalloy) formed into propagation paths for controlling magnetic bubble domains. The bubble domains are formed in a magnetic material such as a garnet film. Conductor loops are located, similar to the permalloy pattern, in close proximity to the garnet film. The conductor loops are used in conjunction with the permalloy elements to perform functions such as generation, annihilation, transfer, replication, and so forth, of magnetic bubble domains when a magnetic field producing current is passed therethrough.
Known single level devices consist of a high resolution permalloy pattern separated from the magnetic garnet film by an insulating spacer. The conductor loops required are produced in the permalloy at the same time as the propagation pattern is formed, thus requiring only one high resolution masking step and eliminating the need for alignment. This method reduces processing time and increases yield, but the circuits produced are generally inferior in performance, reliability and operating life relative to known two level circuits.
Two level devices have a separate conductor layer (usually copper-doped aluminum) in addition to the permalloy propagation pattern. Fabrication starts with deposition of a thin dielectric layer followed by the aluminum conductor layer on the magnetic garnet film. Conductor loops are then delineated using the first high resolution masking step, and defined by etching. Next, the insulating spacer layer and permalloy layer are deposited. Finally, the second high resolution mask is carefully aligned to the conductor pattern and the permalloy propagation pattern is defined.
This two level approach, while more costly, has two distinct advantages over the permalloy only single level approach: (1) the conductor loops are low resistivity aluminum instead of high resistivity permalloy, and (2) the conductor loops can be much closer to the garnet film whereby lower current pulses may be used. The disadvantages of this technique, as currently practiced, are that the range of magnetic bias field over which the circuit may be successfully operated is reduced due to the variation in permalloy-garnet spacing where the permalloy pattern crosses a conductor pattern. In addition, lower fabrication yields are achieved because of the tight registration tolerances required between the levels.